heisenbug

heisenbug: /hi'zen-buhg/ n.  [from Heisenberg's
   Uncertainty Principle in quantum physics] A bug that disappears or
   alters its behavior when one attempts to probe or isolate it.
   (This usage is not even particularly fanciful; the use of a
   debugger sometimes alters a program's operating environment
   significantly enough that buggy code, such as that which relies on
   the values of uninitialized memory, behaves quite differently.)
   Antonym of {Bohr bug}; see also {mandelbug},
   {schroedinbug}.  In C, nine out of ten heisenbugs result from
   uninitialized auto variables, {fandango on core} phenomena
   (esp. lossage related to corruption of the malloc {arena}) or
   errors that {smash the stack}.